This invention relates generally to superconductors and in particular to superconductors resulting from a solid-state reaction between two alloys.
Superconductors are usually compared in terms of critical current densities, J.sub.c, and the critical temperature, T.sub.c. Critical current density values indicate the ability of the material to carry large currents. Values are obtained by dividing the critical current by the cross-sectional area. The critical current is defined as the maximum current passed through a conductor in a transverse magnetic field before a measurable voltage (microvolt/cm.) appears in the conductor.
The critical temperature, T.sub.c is the temperature at which a material achieves the superconducting property. Since the transition from "normal" to superconducting occurs over a temperature range, values for this parameter have been variously reported at the onset of superconductivity or at the midpoint of the temperature range. For the purposes of this application the critical temperature is the midpoint of the range and hence would be lower than the values reported by the other manner.
Intermetallic compounds having an A-15 crystal structure are known to be exceptional superconducting materials. This structure is also referred to as the beta-tungsten crystalline structure. One of the ways in which these compounds are obtained is by a solid-state reaction between two alloys in a vacuum or inert atmosphere at an elevated temperature.
An excellent example of this type of superconductor is the composite of V-9 at.%Ga/V.sub.3 Ga/Cu-17.5 at.%Ga. The critical current density, J.sub.3 has been measured to be 10.1.times.10.sup.6 amps/cm..sup.2 at 4.2.degree. K. and 10 tesla for a composite prepared with V.sub.3 Ga-reaction temperature of 550.degree. C. and a processing time of 400 hours. If the V.sub.3 Ga reaction temperature is increased to 700.degree. C., the processing time is reduced to four hours but the critical-current density is reduced to 1.5.times.10.sup.5 amps/cm..sup.2 at 4.degree. K. and 10 tesla.
One reason why this V.sub.3 Ga composite superconductor has exceptional J.sub.c values is the fine grain size of the V.sub.3 Ga layer. Since the primary flux-pinning sites in an A-15 compound are the grain boundaries of the compound, reducing the grain size increases the critical current density. Factors affecting the grain size are the formation temperature, the amount of the alloying reactant element, e.g., gallium, and the addition of one or more alloying elements.
Unfortunately, the first two factors have disadvantages and the third is largely unknown. A reduced reaction temperature produces a fine grain size but also causes the reaction time to increase to many days as is shown by the above data for the V.sub.3 Ga superconductor. Increasing the alloying reactant accelerates the reaction and decreases the grain size, but also increases the cost of the superconductor due to the high cost of the element and increases brittleness of the composite. This consideration is especially important for V.sub.3 Ga superconductors, which have gallium for the alloying reactant element.
Improvement in the grain size is not the only research objective. Other objectives include increasing the mechanical strength and ductility of superconductors. In addition to the effect that the reaction time and temperature have on grain size, these parameters are also important from economic considerations.